That millions of Syrian civilians are suffering and dying from man-made disaster in the 21st century should prompt moral outrage worldwide.  Syrians believe the world has abandoned them, as they daily fall victim to a cruel campaign of war crimes and crimes against humanity.  If the intent is not genocidal - and it may well be - its effects certainly are.  More than one-third of the 130,000+ killed were civilians, of whom 11,000+ were children.  Over 3 million are cut off from food and humanitarian assistance. Some 9.5 million people have fled their homes, more than 3 million to neighboring countries.  Countless others have been terrorized, traumatized, and tortured.  The conflict is metastasizing to Syria’s neighbors and threatens the security of the region and the world.

Yet the world averts its gaze. Past proclamations that the West would "never again" stand idly by when governments inflicted murder and mayhem on vulnerable populations have, in the case of Syria, proven hollow.  The same excuses that gave cover for inaction in the 20th century are deployed again:  the bloodshed is two-sided, inevitable, and the product of irrepressible internal conflict; outside military intervention for humanitarian ends would  do more harm than good; and national interests can be protected by means  short of humanitarian military intervention. 

Slaughter in Syria has even inspired new alibis: killings elsewhere (Congo) make the choice one of intervening everywhere or nowhere; and the only effective alternative to inaction is to invade and occupy.  One senior White House official reportedly suggested that Syria would be a good place for Iran to have a Vietnam-like experience.  Given the horrible implications of such a scenario for civilians, one hopes it too is merely an alibi for inaction.

The regime of Bashar al-Assad bears the principal responsibility for the 21st century's signature humanitarian abomination.  It is not alone in depredations against the defenseless.  Jihadist elements inspired by regime sectarianism have likewise committed war crimes. 

Yet the Assad clan leads a national government.  It commands an army and an air force.  It uses the military tools of a nation-state - artillery, aircraft, rockets, and missiles - to pound residential areas it cannot or will not capture.  It employs starvation as a weapon.  It commissions criminal gangs to conduct massacres.  It blocks UN humanitarian access to areas it attacks and denies permission for UN relief workers to operate where they must in Syria.     

The international community is not without blame.  Russia and Iran’s military and diplomatic support for Assad  empower him to commit mass crimes against his people.  Gulf money arms  jihadists to wage sectarian battle in Syria. The West’s leverage-free pursuit of a negotiated settlement has failed to stem the fighting (witness the failure of the Geneva II conference).  The result of these policies is to threaten all the countries pursuing them:  the conflict is fanning sectarian hatred throughout the region and well beyond, threatening wider war and the proliferation of terrorism.

For a brief moment in August 2013 the U.S. considered neutralizing the regime's killing machine.  There would have been no invasion, no occupation.  There would have been no slippery slope and no long-term military commitment.  There could have been a straightforward military mission: destroy or significantly degrade the Assad regime's ability to shell and bomb populated areas.  Aircraft, airfields, artillery, rockets, and missiles could have been destroyed.  Lives could have been saved.

Instead the U.S. opted for an agreement permitting the criminal to escape punishment in return for surrendering a weapon that actually has accounted for a tiny percentage of Syria's slaughter.  This inadvertently gave Assad a green light to continue his crimes using non-chemical weapons, such as barrel bombs pushed from helicopters onto schools, hospitals, and bakeries.

Saving Syria's civilians must become the primary focus for concrete international measures.  Secretary-General Ban Ki-moon has been pleading with the regime to permit full, unhindered access to UN relief efforts.  Iran and Russia have a special responsibility to get their client to cooperate and to stop the  war crimes and crimes against humanity.  They should order Assad to stop the slaughter and open Syria to the UN.  Governments in the Arab world must dry up sources of support for jihadists who fight the regime on sectarian grounds and engage in war crimes of their own.  Instead of expecting a Geneva conference to produce miracles, American diplomacy should focus on civilian protection.  If Iran and Russia will not muzzle their client, the credible threat of military strikes on the killing machine must be resurrected.  And those Syrians willing to defend vulnerable populations against regime and jihadist depredations alike should be supported robustly.

The longer we avert our gaze, the greater the peril for all concerned: for Syrians, their neighbors, and us.  "Never again" in the Syrian context should start right now.

Hof is senior fellow at the Atlantic Council's Rafik Hariri Center. Serwer is senior research professor at Johns Hopkins School of Advanced International Studies. Grebowski is in the Future leaders program at the Foreign Policy Initiative.